Traditionally, impurity doping in silicon wafers during the fabrication of microelectronic circuits has been performed by using one of two easily implemented, but relatively shallow, impurity injection methods, namely, diffusion and ion implantation. There are occasions, however, when deep, even wafer-penetrating, vertical doping is desired. For example, in the making of smart-power IC devices, in producing power MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor) with low on-resistance while maintaining high breakdown voltage, and in MEMS (Micro-Electro-Mechanical System) applications.
One way of overcoming these problems has been neutron transmutation doping. (NTD). The NTD process is based on the fact that, although silicon has an atomic number of 14 and an atomic weight of 28, naturally occurring silicon is not entirely made up of the Si.sup.28 isotope. It turns out that Si.sup.29 is present at a concentration of about 4.7 atomic % and Si.sup.30 is present at a concentration of about 3.1 atomic %. Additionally, it turns out that Si.sup.30, when bombarded by thermal neutrons, is transmuted to phosphorus p.sup.31 (atomic number 15). Since the desired level of phosphorus doping is well below the 3.1 at. % of the already present Si.sup.30, it is apparent that a limited amount of neutron bombardment of naturally occurring silicon, will result in the introduction of phosphorus dopant into the silicon. Such phosphorus dopant will be uniformly distributed and will also be in substitutional position in the lattice where it can act as a donor.
While the NTD process has been successfully applied on a number of occasions (see for example Takasu et al. In U.S. Pat. No. 4,910,156), the process does have a number of limitations and shortcomings including (i) neutron beams are hard to focus into a concentrated beam, (ii) the neutron flux can make surrounding equipment radioactive, and (iii) neutron beams, in practice, have a flux around 10.sup.14 /cm.sup.2 whereas a focused proton beam can have a flux anywhere between about 10.sup.13 to 10.sup.17 /cm.sup.2. As will be discussed in detail below, proton induced nuclear transmutation doping achieves the same end goals as NTD (namely deep, even wafer-penetrating, vertical and uniform n-type doping) but without some of the aforementioned disadvantages. We will refer to this process by its more general name--ion transmutation doping (ITD).
Prior to the work that led up to the present invention, the possibility of proton induced ITD on silicon received little or no attention. In particular, detailed information regarding the mechanisms and nuclear reaction cross-sections, as well as side effects, was not available for practical use. We were unable to find prior art relating to deep and practical proton transmutation doping. Of interest were several references that teach the use of conventional doping methods in conjunction with proton bombardment for the purpose of creating lattice damage and hence a high density of recombination sites. Examples of these include Dixon et al. (U.S. Pat. No. 4,124,826), Adam et al. (U.S. Pat. No. 4,806,487) and Voss (U.S. Pat. No. 4,987,087). As will be explained below, a transitory side effect of proton induced ITD is the formation of some sulfur. The use of sulfur compounds in majority carrier devices is explored by Barron et al. (U.S. Pat. No. 5,760,462), suggesting that sulfur could actually be used to advantage although in our case any such byproduced sulfur would be likely, at a minimum, to damage any iron containing equipment with which it came in contact. A method for removing this sulfur byproduct is included as part of the present invention.